The objective of this study is to evaluate the efficacy of 256U87 (the prodrug of acyclovir) compared to placebo and standard acyclovir therapy in the treatment of a single episode of recurrent genital herpes in immunocompetent patients. There is a 90% chance that the patient will receive an active drug. Efficacy variables include the length of episode, the time to lesion healing, the duration of viral shedding, the duration and severity of pain/discomfort, and the proportion of patients with aborted episodes